Not applicable.
The invention relates to the field of implantable articles. More particularly, the invention relates to a prosthesis system which has an expandable stem region to secure the prosthesis system within a long bone.
Joint replacement surgery is quite common and enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints are normally composed of metallic and/or ceramic components that are fixed to existing bone closest to the joint being replaced.
Artificial hip joints, for example, include several components. A femoral component of an artificial hip includes an elongate stem or shaft at its distal end that is affixed within the medullary canal of the femur. A proximal end of the stem has a neck region which includes a trunnion, to which is attached a femoral head. The acetabular shell is a separate component of an artificial hip joint that is affixed within existing bone such as the acetabulum. The acetabular shell often includes a cup-like liner that receives the femoral head.
Typically, for a hip joint, the elongate stem or shaft portion of the prosthesis is implanted in the medullary canal of the femur. However, many times the femur or other long bone where the stem portion is to be secured can be in a damaged or weakened condition insufficient to support the prosthesis. In such cases, it may be necessary to use a stem having a length that extends beyond the isthmus or narrow portion of the bone.
However, extending the prosthesis beyond the isthmus poses a conflicting problem since the supporting bone beyond the isthmus both widens and can diverge in more than one plane. A prosthesis stem narrow enough to pass through the isthmus may not have sufficient dimensions to provide a secure fit beyond the isthmus where the supporting cortical bone widens or diverges in more than one plane.
Thus it would be desirable to have a prosthesis system that has a strong and effective support for structurally unsound long bone while still being able to pass through the narrow portion of the long bone.
The invention provides a modular joint prosthesis which includes an implantable elongate stem member having a bore extending therethrough. The stem member has a proximal end and a distal end with at least one longitudinally oriented slot formed in the distal end. The slot is longitudinally formed in at least a portion of the stem member, and it extends from the distal end towards the proximal end to form at least two tines at the distal end of the stem.
The prosthesis also includes a bolt member that is disposable within the bore of the stem member. The bolt member has a proximal end and a distal end, at least a portion of which is threaded.
An expander nut is disposed between the tines of the prosthesis and it is threadably matable with the distal end of the bolt member. The rotation of the bolt member in a first direction is effective to draw the expander nut towards the proximal end of the stem member to separate the tines, thus expanding the distal end of the stem member. The expander nut may have more than one diameter to spread the tines to different diameters and alternatively, the tines may be provided with varying thicknesses.